In cased wellbore operations, it is typical to use two or more concentric casings which decrease in diameter with increased wellbore depth. Perforation guns may be used during production and abandonment after production has ceased. Production requires perforations in the inner most casing of the concentric casings which has the smallest diameter and is located at the largest depth (downhole) of the wellbore relative to the other casings. Abandonment requires perforations in casings that typically have larger diameters and are located at a shallower depth (uphole) relative to the perforations made during production.
For example, during a cased wellbore completion process, a gun string assembly is positioned in an isolated zone in the wellbore casing. The gun string assembly comprises a plurality of perforating guns coupled to each other using connections such as threaded tandem subs. The perforating guns are then fired, creating holes through the casing and the cement and into the targeted rock. These perforating holes then allow fluid communication between the oil and gas in the rock formation and the wellbore. During the completion of an oil and/or gas well, it is common to perforate the hydrocarbon containing formation with explosive charges to allow inflow of hydrocarbons to the wellbore. These charges are loaded in a perforation gun and are typically “shaped charges” that produce an explosively formed penetrating jet that is propelled in a chosen direction, when detonated. When a charge in a perforating gun system is detonated and the well perforated, entrance holes are created in the well casing and explosives create a jet that penetrates into the hydrocarbon formation. The diameter of the entrance hole depends on several factors including but not limited to the nature of the liner in the shaped charge, the explosive type, the thickness and material of the casing, the water gap in the casing, centralization of the perforating gun, number of charges in a cluster and number of clusters in a stage. The term “water gap” used herein is a clearance between the outer diameter of a perforating gun and the inside diameter of a casing.
Perforation also takes place after production has ceased and the wellbore is prepared for abandonment. For example, during abandonment operations, often referred to as “plug-and-abandonment,” perforations are required to open a section of the casing in order to deposit a sealant, such as cement. This is intended to prevent fluids from production in the downhole casing from migrating toward the surface where it could potentially contaminate water tables. Adequate perforation creates as many large openings in the intended section of casing as possible. In order to achieve this, a selected interval may be perforated with a gun system, followed by removal of the gun system. It is desired to have an improved gun system that is able to achieve adequate openings in the casing that is time and cost effective.